In making circuit boards and the like, it is frequently routine to apply a uniform coating over the workpiece, for example, by roller coating, curtain coating, screen printing, and the like. A preferred system for this involves electrostatic spraying, which permits application of a uniform coating under automated assembly line conditions. So far as I am aware, electrostatic spray techniques have not been used to apply uniform coatings of 100% solids (i.e., liquid but solventless) curable solder masks, plating resists, etch resists, and the like. In a production operation the workpiece to be electrostatically spray coated is placed automatically on an endless belt, which may take it through one or more preliminary operations (cleaning, bottom-coating to enhance conductivity, etc.) before it enters the spray chamber. A current assembly line operation is shown, e.g., in Circuits Manufacturing, February 1986, pp. 80 and 81, for electrostatically spray-coating both sides of circuit boards; the operation includes UV printing and development in dilute sodium carbonate solution.
In all currently known board spray systems, overspray inevitably collects on the moving belt, between the boards, along the belt sides, and even off the belt in areas paralleling the belt. This overspray is difficult to deal with. It drips from the chamber walls and from the belt. Some sticks to the belt, where it tends to build up in uneven layers. This results in random loss of conductivity (from the workpiece to the ground), which in turn results in non-uniform coatings, transfer of spray material to the underside of the panels, and other problems. One of the best means of coping with overspray in the past has involved placing paper under the workpieces. The paper catches the overspray, and the spattered paper is retrieved and discarded. This solution has obvious drawbacks. Means have to be provided for feeding the paper properly to the system and recovering disposing of it. Spray loss, paper costs, and labor costs add to the expense.